This invention relates to an electrophotographic photosensitive member, more particularly to a high sensitivity electrophotographic photosensitive member comprising a photoconductive layer composed primarily of a microcrystalline silicon.
As the materials constituting photoconductive layers of electrophotographic photosensitive members of the prior art, there have been known inorganic materials such as CdS, ZnO, Se, Se-Te, amorphous silicon (a-Si) and the like and organic materials such as poly-N-vinyl-carbazole (PVCz), trinitrofluorenone (TNF) and the like. However, these photoconductive materials involve various problems as materials to be used in practical application and under the present circumstances these materials have been used in various applications depending on the situations, more or less at the sacrifice of the characteristics of the system. For example, Se and CdS are materials which are essentially harmful to human bodies and therefore it is required to have a particular concern about safety in preparation of these. For this reason, the production device tends to be complicated, requiring a superfluous expenditure for preparation thereof. It is also required to recover such a material as Se and the expense necessary for such a recovery will be reflected in the material cost. As for characteristics, in case of, for example, Se (or Se-Te system), the crystallization temperature is as low as 65.degree. C. and therefore crystallization may occur during repeated copying, thereby tending to cause practical problems with respect to residual images or others and resulting ultimately in the drawback of short life. On the other hand, in case of ZnO, physical properties of the material are susceptible of oxidation and reduction, thus tending to be markedly influenced by the environmental atmosphere, and hence it has the problem of lower reliability. Further, in case of organic photoconductive members, PVC and TNF have recently been questioned about their possibility as carcinogens and therefore it is quite probable that there may be invited the situation to prohibit production of these materials. Besides, since they are organic materials, they suffer from poor thermal stability and weak abrasion resistance, having thus the drawback of shorter product life.
Meanwhile, an amorphous silicon (a-Si) has recently attracted attention as an inorganic material, and there are so many attempts to utilize such a material for a solar cell as well as various investigations about other applications such as a photoconductive material for an electrophotographic photosensitive member. Such an amorphous silicon material has the advantages as an electrophotographic photosensitive member which are deficient in other materials as mentioned above. That is, (1) it is a non-polluting material; (2) it has a better photosensitivity to the light in the longer wavelength region than the materials of the prior art; and (3) it has a high surface hardness and excellent in abrasion resistance. On account of such advantages, it is greatly expected to be applicable for an electrophotographic photosensitive member (see U.S. Pat. Nos. 4,225,222 and 4,265,991).
However, the amorphous silicon layer can be formed at a very small rate so that it takes a very long period of time to produce a drum, whereby productivity is disadvantageously low as pointed out in the art (see for example Japanese Provisional Patent Publication No. 86341/1979). That is, an amorphous silicon is generally formed according to the high frequency glow discharge decomposition method using silanes as the starting material, but the film forming rate is remarkably small on the order of 4 .ANG./sec, and for example, the reaction time as long as 13 hours or more is required for formation of an amorphous silicon with a thickness of about 20 .mu.m. It is therefore indeed necessary to enhance greatly the film forming rate for the purpose of realizing commercial application of an amorphous silicon for a photoconductive member of an electrophotographic photosensitive member. However, when it is intended to accelerate the film forming rate of an amorphous silicon, the resultant film will be predominated by the binding structures such as (SiH.sub.2).sub.n, SiH.sub.3, etc. as apparently seen from IR absorption spectrum thereof. Further, due to higher content of voids, silicon dangling bonding will be increased. As the result, photoconductivity is worsened to make the product difficultly available as an electrophotographic photosensitive member. For this reason, the film forming rate of an amorphous silicon layer is limited to 10 .ANG./sec at the highest, generally about 4 .ANG./sec as mentioned above. But such a low film forming rate is now a great drawback in industrial application, since it will bring about low productivity which may be one factor leading to increase in cost of the product drum. Also, a long wavelength photosensitive member corresponding to a semiconductor laser light source may be photosensitive to the wavelength region up to approximately 850 nm, but an amorphous silicon has the drawback that it has no sufficient sensitivity in the long wavelength region due to the relation to its light absorption coefficient.